Regression in 1.9.1: Arrays & Hashes now become nil

[zarqman]

Hi there! It appears that #47 (released in v1.9.1) causes a change from previous behavior.

In v1.9.0 & prior, converting .blank? -> nil was implicitly limited to Strings because those were conditional on if value.respond_to?(:strip).

#47 separated the .blank? test from respond_to?(:strip) which now causes non-Strings, such as Hashes and Arrays, to also be converted to nil.

This PR adds a check that an attribute is a String before running strip_string(), which matches the prior behavior.

Side note: this would likely allow many of the other respond_to?() tests to be dropped, but that seemed out of scope to be including here.

[swordray]

Breaks false too.

[rmm5t]

@zarqman Thank you for this!! I adjusted your commit a bit and merged it into master here:
20a2aa9

I also just release the fix a v1.9.2. Sorry about the mishap.
https://rubygems.org/gems/strip_attributes