fix multipart/form-data without filename and content type

[larryhu]

Sometime just want send fields without file when content-type is multipart/form-data

[codecov]

Codecov Report

Merging #236 into v2 will decrease coverage by 0.25%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##               v2     #236      +/-   ##
==========================================
- Coverage   95.81%   95.56%   -0.26%     
==========================================
  Files           9        9              
  Lines        1076     1082       +6     
==========================================
+ Hits         1031     1034       +3     
- Misses         24       25       +1     
- Partials       21       23       +2

Impacted Files	Coverage Δ
util.go	90.64% <66.66%> (-1.84%)	⬇️

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[moorereason]

Please add tests to this PR.

[jeevatkm]

@larryhu Thank you for targeting v2 branch. Could you please add test cases for the PR?

Thanks @moorereason

[jeevatkm]

@larryhu ping for unit test cases.

[jeevatkm]

It seems like contributor @larryhu didn't get a chance to add a unit test case for this PR. So I'm merging in and adding a unit test. Kindly ensure next time, thank you!