s3put: accept headers values containing "="

s3put tries to split headers on the "=" sign. Some headers include a "=" sign (e.g. max-age), and can't be used. 

This pull request splits on the first equal sign to support those.

Caveat: if a user uses the command correctly (`s3put --header "header1=value1 header2=value2"), they won't get an exception.
Merge pull request #1700 from krallin/s3put-fix-header

s3put: accept headers values containing "=". Fixes #1700.