Make content_type_mappings option more user-friendly

[svyatov]

Is your feature request related to a problem? Please describe.
I spent quite some time debugging an issue in our project due to misconfiguration of content_type_mappings - keys were strings instead of symbols.

Describe the solution you'd like
I'd like for content_type_mappings option to be more user-friendly and/or fool-proof. Two options here:

1. Raise an error when there are string keys provided;
2. Don't raise any errors and make this option a "hash with indifferent access".

Additional context
This issue was discovered when we tried to upload .json files. Linux file utility returns text/plain content type while content type from extension is application/json. So in order to fix this we had to add a custom mapping:

Paperclip.options[:content_type_mappings] = {
  'json' => 'text/plain'
}

The above code didn't work, because the key is a string, not a symbol. The following code works as expected:

Paperclip.options[:content_type_mappings] = {
  json: 'text/plain'
}

This happens because of this code:

kt-paperclip/lib/paperclip/media_type_spoof_detector.rb

Lines 86 to 88 in f036773

	def filename_extension
	File.extname(@name.to_s.downcase).sub(/^\./, "").to_sym
	end

There was a related issue in the original repo: thoughtbot#2425

I'm happy to provide a PR once we settle on the exact solution: raise or accept both strings and symbols. Plus we could fix the case-sensitivity issue mentioned above as well.

[ssinghi]

Thank you @svyatov!
I think the code should support both strings and symbols, please create a PR for this. Thanks!