fix(config)!: change content_type from endpoint config

[shouya]

Previously the content_type in endpoint config is used to enforce the interpretation of the request body in a certain content type. This feature is useful on some feeds that reports wrong content types.

Later I discovered that this feature is more generally needed. For example, in the upcoming merge filter, we may also want to assume the content_type for the merged source. So in this PR, I moved the content_type field from endpoint's config to the client's config. I also renamed it to assume_content_type to make it more clear.

BREAKING CHANGE: This change will break the existing configuration if you use content_type field in an endpoint. You need to update your endpoint config as follows:

# old
endpoints:
  - path: /foo
    source: http://website/feed.xml
    content_type: application/atom+xml
    filters: ...

# new
endpoints:
  - path: /foo
    source: http://website/feed.xml
    client:
      assume_content_type: application/atom+xml
    filters: ...