Add size to Map class

[miyucy]

No description provided.

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[miyucy]

@googlebot I signed it!

[JasonLunn]

@miyucy - could you add a unit test?

[miyucy]

@JasonLunn I'm sorry, it my fault. I just added simple tests. Could you review again?

[JasonLunn]

Good test! The unit test you've added shows that the proposed change appears to be missing an equivalent implementation in the JRuby equivalent of map.c

[miyucy]

@JasonLunn Thanks for review. I wasn't look JRuby build log.. I added annotation like RubyRepeatedField.java. Could you review it again?

[JasonLunn]

LGTM

[acozzette]

Thanks, @miyucy.