Fix JRUBY-5550: Using shoulda prevents Rails from running tests in ru…

…by1.9 mode

Array#join under 1.9 mode tries to coerce non-strings using to_str. NoMethodError is ignored, but in our case we ignored it and left $! set. This prevented Miniunit from running tests.

src/org/jruby/RubyClass.java

@@ -633,6 +633,8 @@ public IRubyObject finvokeChecked(ThreadContext context, IRubyObject self, Strin
                     if(self.respondsTo(name)) {
                         throw e;
                     } else {
+                        // we swallow, so we also must clear $!
+                        context.setErrorInfo(context.nil);
                         return null;
                     }
                 } else {