Problem: Hash#fetch doesn't raise KeyError when a key cannot be found

Solution: change the Exception class raised to KeyError when a key
cannot be found.

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

@@ -130,7 +130,7 @@ class Hash
       elsif none != NONE
         none
       else
-        raise RuntimeError, "Key not found: #{key}"
+        raise KeyError, "Key not found: #{key}"
       end
     else
       self[key]

mrbgems/mruby-hash-ext/test/hash.rb

@@ -79,7 +79,7 @@ assert('Hash#fetch') do
   begin
     h.fetch("gnu")
   rescue => e
-    assert_kind_of(StandardError, e);
+    assert_kind_of(KeyError, e);
   end
 end