multipart: more lenient content-type parsing

It's not uncommon for files to have mime types like `image/svg+xml`.
The + in there was tripping up the strictly-`@ta` parser for the content-type.

Here we change the parser to accept +, and all the other characters that the
`@t` parser accepts. This is perhaps a bit overkill, but we primarily care
about getting the parsed request to the caller. If any further validation is
required, it can happen upstream.

Fixes #5282.

pkg/garden/lib/multipart.hoon

@@ -37,7 +37,7 @@
   ++  cut   (jest 'Content-Type: ')
   ++  dim   (jest (cat 3 '--' del))
   ++  nip   (jest '\0d\0a')
-  ++  nab   (more fas urs:ab)
+  ++  nab   (more fas (cook (cury rap 3) (plus qit)))
   ++  nag   (dine ;~(less ;~(plug nip dim) next))
   ++  nod   (dine ;~(less doq next))
   ++  nom   (dine alp)