Compress all wheel content with explicit ZIP_DEFLATED (#715) (#838)

https://docs.python.org/2/library/zipfile.html#zipfile.ZipFile.writestr

Note: When passing a ZipInfo instance as the zinfo_or_arcname parameter,
the compression method used will be that specified in the compress_type
member of the given ZipInfo instance. By default, the ZipInfo
constructor sets this member to ZIP_STORED.