Check modifiable for String `bang' methods

mrbgems/mruby-string-ext/mrblib/string.rb

@@ -95,6 +95,7 @@ class String
   #    "hello".lstrip!      #=> nil
   #
   def lstrip!
+    raise RuntimeError, "can't modify frozen String" if frozen?
     s = self.lstrip
     (s == self) ? nil : self.replace(s)
   end
@@ -111,6 +112,7 @@ class String
   #    "hello".rstrip!      #=> nil
   #
   def rstrip!
+    raise RuntimeError, "can't modify frozen String" if frozen?
     s = self.rstrip
     (s == self) ? nil : self.replace(s)
   end
@@ -123,6 +125,7 @@ class String
   #  <code>nil</code> if <i>str</i> was not altered.
   #
   def strip!
+    raise RuntimeError, "can't modify frozen String" if frozen?
     s = self.strip
     (s == self) ? nil : self.replace(s)
   end
@@ -183,6 +186,7 @@ class String
   #    string                  #=> "thsa sting"
   #
   def slice!(arg1, arg2=nil)
+    raise RuntimeError, "can't modify frozen String" if frozen?
     raise "wrong number of arguments (for 1..2)" if arg1.nil? && arg2.nil?
 
     if !arg1.nil? && !arg2.nil?

mrblib/string.rb

@@ -78,6 +78,7 @@ class String
   #
   # ISO 15.2.10.5.19
   def gsub!(*args, &block)
+    raise RuntimeError, "can't modify frozen String" if frozen?
     str = self.gsub(*args, &block)
     return nil if str == self
     self.replace(str)
@@ -123,6 +124,7 @@ class String
   #
   # ISO 15.2.10.5.37
   def sub!(*args, &block)
+    raise RuntimeError, "can't modify frozen String" if frozen?
     str = self.sub(*args, &block)
     return nil if str == self
     self.replace(str)