Refine `Content-Type` negotiation

[trevor-scheer]

The parseBody function is only using startsWith and endsWith to figure the content type.

datasource-rest/src/RESTDataSource.ts

Lines 288 to 289 in 5ac9b52

	(contentType.startsWith('application/json') ||
	contentType.endsWith('+json'))

In most cases (application/json) this is fine. But for users depending on the +json condition, this breaks for cases like this one:

application/ld+json; charset=utf-8

Ref: #228 (reply in thread)