fix: accept text/plain data

[rchl]

Sentry is making requests with text/plain content-type which fails to parse without text/plain handler.

Sentry documentation states:

Envelope requests may contain all headers as regular store requests. The only accepted content-type is application/x-sentry-envelope, which is implied if it is missing. To minimize the necessity for CORS preflights it's acceptable to send text/plain, multipart/form-data and application/x-www-form-urlencoded as well. In either of those cases the behavior however is the same as using application/x-sentry-envelope.
https://develop.sentry.dev/sdk/envelopes/

It's a bit confused about the use of "the only accepted" but otherwise should be clear that text/plain is also allowed.

Resolves #104