Enable JSON Beautifier when JSON type is stated, but not inferred

[cnotin]

I have a few examples of JSON responses, where the JSON content-type is stated and displayed as such in Burp Proxy "MIME Type" column, and where the JSON content is also valid, but where JSON Beautifier tab is missing. After a quick debugging session I see that the stated MIME type is indeed JSON, but the inferred MIME type (by Burp) is empty... It could be a Burp issue too. This PR enables JSON Beautifier tab if either (stated/inferred) is JSON.

I'm using Burp Pro, latest version (2.1.05)