Check for undefined body

[hogpilot]

Checks if init.body is defined before converting it to a string in axiosFetch method.

Fixes #63

[coveralls]

Pull Request Test Coverage Report for Build 270

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 267:	0.0%
Covered Lines:	16
Relevant Lines:	16

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💛 - Coveralls

[mdlavin]

@hogpilot Thank you for contributing a fix and your change looks good to me. Would you mind adding a testcase to make sure that it doesn't get regressed by future changes?

[mdlavin]

@hogpilot Double thanks for the fix and test. I'll wait on the update on the PR and will release a new version after that one merges too

[hogpilot]

@mdlavin Thanks for your quick review. I appreciate the test reminder

[mdlavin]

This fixe long with #62 should be included in @lifeomic/axios-fetch@1.5.2