tests: fix expected filename in TestRequestBodyMultipartFile

[clobrano]

trafficstars

currently TestRequestBodyMultipartFile expect the filename given by

func (*multipartPart) FileName() string

to be a fullpath in one of the test cases, however the documentation states that:

the filename is passed through filepath.Base (which is platform dependent) before being returned.

then the test fails.

This change fix the expected filename for this test, using the same filepath.Base function.

Fixes: #103

[aries-zhang]

Hi @clobrano @EstebanJesus any chance this PR and all other PRs that are failed due to this can be merged sooner?

Maybe also @gavv ?

[EstebanJesus]

Hi @aries-zhang, I really don't know what the strategy is like, but I don't see a problem with it.

[clobrano]

Hey, let me know if there is more I can do to improve this PR.

[gavv]

Hi, sorry for delay! Thanks!

[gavv]

Follow-up commit a54cf58c872b620d00b71cc961822765ae316d64 to fix the build completely