feat(put multipart): add parsing for multipart content with methods other than POST

[dariok]

As described in #104, eXist does not currently parse multipart content for anything but POST requests.
When PUTing multipart content, roaster will fail as it cannot find the required parameters.

This PR adds parsing of multipart content for other request types, creating a structure basically identical to that of eXist’s native parsing.

[dariok]

This patch is still necessary in eXist 7.0.0-Beta3 due to several problems in both eXist and roaster.

• [BUG] Error trying to access binary data from EXPath HTTP response eXist-db/exist#5030 means that binary uploads are actually unusable
• upload formats other than binary ignore any information from Content-Disposition, e.g. filename (and, of course, do not provide a file size either)
• both eXist and roaster 1.12.2 ignore additional header information, e.g. a Content-Type for the uploaded file.

[dariok]

Actually, this doesn’t help any more as everything’s just too broken.

With eXist 7.0.0Beta3, if a multipart-upload has been made, request:get-data() will always be empty (checked in router.xql:90). This means, I cannot parse the data that has been uploaded.
I guess this is connected to issue 5030 (linked above) and because eXist has already tried to parse the multipart data.

This means, I cannot parse the data that has been uploaded and create a useful map.

But, as said, eXist doesn’t really do a good job of parsing multipart uploads.
I will open an issue for eXist and revert to eXist 6.4.1 for the time being where this patch still works.

[dariok]

To track the upstream issues in eXist, I opened new issues there:

• [BUG] parsing of multipart does not work correctly for PUT eXist-db/exist#6580
• [BUG] auto-parsing of multipart-formdata is incomplete eXist-db/exist#6578

When both are fixed, this PR will be obsolete.

As the request data are gone due to the attempt at parsing (cf. eXist-db/exist#1536 (comment)), there is not way to PUT data.