Make extensions and content types white/blacklists take a string, regexp, or array of values

[mehlah]

Writing

def content_type_whitelist
  [/image\//]
end

isn't very convenient or intuitive due to the array.

This PR suggests to let #content_type_whitelist and extension_whitelist takes either a
string, a regexp, or an array of values (same thing for blacklists).

[thomasfedb]

For consistency, can you add a space in Array[String,Regexp] so that it becomes Array[String, Regexp].

[thomasfedb]

Can you rename the relevant specs to be in keeping with the names for your new specs, eg: "accepts a single extension string value" would have the counterpart "accepts an array of extension string values".

Or even better, something like:

context "when there is a blacklist" do
  context "when the backlist is a single value" do
    # ...
  end

  context "when the backlist is an array of values" do
    # ...
  end
end

[mehlah]

@thomasfedb Thanks for your comments. I addressed those to make the specs perfectly symmetric and rebased against master.

[thomasfedb]

Thanks @mehlah! ✨