Add support for application application/x-gzip, equivalent of `appl…

…ication/gzip` since some servers still use this type of content-type header (#89)

reference:
https://stackoverflow.com/questions/21870351/difference-between-x-gzip-and-gzip-for-content-encoding
https://mimetype.io/application/x-gzip

src/mime_types.rs

@@ -450,7 +450,7 @@ pub static MIME_TYPES: &[(&str, &[&str])] = &[
     ("gv", &["text/vnd.graphviz"]),
     ("gxf", &["application/gxf"]),
     ("gxt", &["application/vnd.geonext"]),
-    ("gz", &["application/gzip"]),
+    ("gz", &["application/gzip", "application/x-gzip"]),
     ("h", &["text/plain"]),
     ("h261", &["video/h261"]),
     ("h263", &["video/h263"]),