helpers for content type checking

[sckott]

via https://github.com/ropensci/pangaear/issues/68 - seems like a method on the HttpResponse could be helpful to check response content types:

• accounting for any http rules
• with options for what to do when the content type does not match

[sckott]

pseudo-code

x <- HttpClient$new('https://httpbin.org/get?foo=bar')

maybe methods like $raise_for_status(), but for the most common content types

x$raise_for_ct_xml() # == text/xml
x$raise_for_ct_json() # == application/json
x$raise_for_ct_html() # == text/html

where you could choose to stop or warn

x$raise_for_ct_json(condition = "warning") # options: stop, warning

or you could pass any string to customize the match

x$raise_for_ct("application/vnd.citationstyles.csl+json")