Setting a key using send should return the new value

[tjwp]

There seems to be an unnecessary line at the end of internal_write_key
that results in send("#{key_name}=", value) returning nil. This breaks
compatibility with 0.12.

The @attributes variable that is set to nil in the last line of
internal_write_key does not appear to be
referenced elsewhere.

[tjwp]

The Travis failure for JRuby looks unrelated to this change.

[cheald]

IIRC, @attributes is relevant to ActiveModel - I'll see what I can figure out, but I don't think it's valid to just remove it.

[mtsmfm]

@cheald Sorry, could you tell me why is this @attributes needed?
I'm trying to change MM to support newer version of ActiveModel and I'm wondering if we can remove that because in Rails 5.2 it breaks ActiveModel attribute behavior.

[cheald]

I honestly don't recall; if you can get it passing spec suites without it, then go nuts! I seem to think it was something that fulfilled an ActiveModel requirement, though I couldn't tell you why.