Always add trailing .0 in Float#inspect; ref #4225

Trailing `.0` is removed from `Float#to_s` and `Float#inspect` at
9d08025b. However, I think the more human-readable format is better
for `Float#inspect`.

For example, in the `Float#to_s` format, the failure message is not
well understood when testing values including types by `eql?` (e.g.
`Numeric#step` test).

  ```ruby
  assert "example" do
    exp = 1.0
    act = 1
    assert_operator(exp, :eql?, act)  #=> Expected 1 to be eql? 1.
  end
  ```

src/numeric.c

@@ -203,19 +203,8 @@ integral_coerce_step_counter(mrb_state *mrb, mrb_value self)
  *  representation.
  */
 
-/* 15.2.9.3.16(x) */
-/*
- *  call-seq:
- *     flt.to_s  ->  string
- *
- *  Returns a string containing a representation of self. As well as a
- *  fixed or exponential form of the number, the call may return
- *  "<code>NaN</code>", "<code>Infinity</code>", and
- *  "<code>-Infinity</code>".
- */
-
 static mrb_value
-flo_to_s(mrb_state *mrb, mrb_value flt)
+flo_to_str(mrb_state *mrb, mrb_value flt, mrb_bool add_dot_zero)
 {
   mrb_float f = mrb_float(flt);
   mrb_value str;
@@ -258,6 +247,9 @@ flo_to_s(mrb_state *mrb, mrb_value flt)
       str = mrb_float_to_str(mrb, flt, fmt);
       goto insert_dot_zero;
     }
+    else if (add_dot_zero) {
+      mrb_str_cat(mrb, str, ".0", 2);
+    }
 
     goto exit;
   }
@@ -267,6 +259,49 @@ flo_to_s(mrb_state *mrb, mrb_value flt)
   return str;
 }
 
+/* 15.2.9.3.16(x) */
+/*
+ *  call-seq:
+ *     flt.to_s  ->  string
+ *
+ *  Returns a string containing a representation of self. As well as a
+ *  fixed or exponential form of the number, the call may return
+ *  "<code>NaN</code>", "<code>Infinity</code>", and
+ *  "<code>-Infinity</code>".
+ *
+ *  Trailing <code>.0</code> is removed.
+ *
+ *     3.0.to_s   #=> 3
+ *     3.25.to_s  #=> 3.25
+ */
+
+static mrb_value
+flo_to_s(mrb_state *mrb, mrb_value flt)
+{
+  return flo_to_str(mrb, flt, FALSE);
+}
+
+/*
+ *  call-seq:
+ *     flt.inspect  ->  string
+ *
+ *  Returns a string containing a representation of self. As well as a
+ *  fixed or exponential form of the number, the call may return
+ *  "<code>NaN</code>", "<code>Infinity</code>", and
+ *  "<code>-Infinity</code>".
+ *
+ *  Trailing <code>.0</code> is added.
+ *
+ *     3.0.to_s   #=> 3.0
+ *     3.25.to_s  #=> 3.25
+ */
+
+static mrb_value
+flo_inspect(mrb_state *mrb, mrb_value flt)
+{
+  return flo_to_str(mrb, flt, TRUE);
+}
+
 /* 15.2.9.3.2  */
 /*
  * call-seq:
@@ -1684,7 +1719,7 @@ mrb_init_numeric(mrb_state *mrb)
   mrb_define_method(mrb, fl,      "eql?",      flo_eql,        MRB_ARGS_REQ(1)); /* 15.2.8.3.16 */
 
   mrb_define_method(mrb, fl,      "to_s",      flo_to_s,       MRB_ARGS_NONE()); /* 15.2.9.3.16(x) */
-  mrb_define_method(mrb, fl,      "inspect",   flo_to_s,       MRB_ARGS_NONE());
+  mrb_define_method(mrb, fl,      "inspect",   flo_inspect,    MRB_ARGS_NONE());
   mrb_define_method(mrb, fl,      "nan?",      flo_nan_p,      MRB_ARGS_NONE());
 
 #ifdef INFINITY

test/t/float.rb

@@ -239,6 +239,11 @@ assert('Float#to_s') do
   end
 end
 
+assert('Float#inspect') do
+  assert_equal("-3.25", -3.25.inspect)
+  assert_equal("50.0", 50.0.inspect)
+end
+
 assert('Float#eql?') do
   assert_operator(5.0, :eql?, 5.0)
   assert_not_operator(5.0, :eql?, 5)