fix source report artifact error

[malaskowski]

Fixed error at report on source comparison report:

/report/assets/libs/angular/angular.js:13550 SyntaxError: Unexpected token < in JSON at position 0
    at JSON.parse (<anonymous>)
    at fromJson (/report/assets/libs/angular/angular.js:1321)
    at defaultHttpResponseTransform (/report/assets/libs/angular/angular.js:10393)
    at /report/assets/libs/angular/angular.js:10484
    at forEach (/report/assets/libs/angular/angular.js:322)
    at transformData (/report/assets/libs/angular/angular.js:10483)
    at transformResponse (/report/assets/libs/angular/angular.js:11278)
    at processQueue (/report/assets/libs/angular/angular.js:15961)
    at /report/assets/libs/angular/angular.js:15977
    at Scope.$eval (/report/assets/libs/angular/angular.js:17229)(anonymous function) @ /report/assets/libs/angular/angular.js:13550
artifacts.service.js:62 Failed to load artifact! Object SyntaxError: Unexpected token < in JSON at position 0(…)handleFailed @ artifacts.service.js:62

The reason is invalid content type of source artifact (which was set to application/json - fixed also).
It is also not necessary downloaded which was also fixed.

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[wiiitek]

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