Add Decompress helper functions to "String" type

String type could have gzip helper functions such as "IsGzip" and "GzipDecompress()" functions for use as below:
```
he.GET("/data/abc.bin").
WithHeader("Range", "bytes=0-2000").
Expect().
Status(http.StatusOK).
HasContentType("application/octet-stream").
Body().
GzipDecompress().
IsEqual(expectedData[:2000])
```
Or alternatively a generic decompress method that takes compression type as parameter:
```
he.GET("/data/abc.bin").
WithHeader("Range", "bytes=0-2000").
Expect().
Status(http.StatusOK).
HasContentType("application/octet-stream").
Body().
Decompress("gzip").
IsEqual(expectedData[:2000])
```