async does not accept some valid formats in multipart/form-data request.

[thorikawa]

There are some cases that async does not parse properly for multipart/form-data request.

e.g.
According to the code, async searches "Content-Disposition" header in a case sensitive way, but it is case insensitive according to RFC.

Also, according to this line async expects "Content-Type" line is located after "Content-Disposition" line, but the order is not strict, so it can be reverted.