net/http: remove discrepancies between the MIME Sniffing Spec and its implementation
The change fixes the following deviations between the existing implementation and the Spec:
1. Using pattern instead of "mask" for assertion and iteration in the Pattern Matching Algorithm.
2. Rename "image/vnd.microsoft.icon" to "image/x-icon" and add another signature for the same.
3. Using named strings instead of hexadecimal representation in "application/zip" and "application/x-rar-compressed".
4. Reordering "sniffSignatures" in accordance with the Spec section "Identifying a resource with an unknown MIME type".
In addition to the above fixes, unit tests for Image MIME type group are added.