Strip whitespace when looking up ManyToMany fields

[raghavsethi]

This makes imports more robust when users enter whitespaces after
commas.

[raghavsethi]

Sorry, I couldn't find the develop branch here https://github.com/django-import-export/django-import-export/branches/all what am I missing?

[raghavsethi]

@bmihelac: Would love to know if this is something you want to merge quickly (after I address all your feedback, of course), if not I can subclass ManyToManyWidget for my current project.

[manelclos]

LGTM

[bmihelac]

Merged, thanks @raghavsethi for patch, @manelclos for reviewing

[mgrdcm]

Uh oh @bmihelac! This is causing tests to fail: https://travis-ci.org/django-import-export/django-import-export/jobs/302923330

[bmihelac]

looking into it, thanks for noticing

[bmihelac]

Fix in d6615a7, sorry for inconvinience

[raghavsethi]

Sorry about that! Thanks for the fix!

[mgrdcm]

Sorry @bmihelac I would have fixed but was looking at the Travis-CI issue instead :)

[bmihelac]

No problem @raghavsethi, I checked the code and was confident, travis would warn if anything is wrong.
@mgrdcm it is right for me to fix, since i broke it :)