Ignore header case to determine if content is gzipped (#143)

Summary:
This PR updates the Network plugin's response parser to ignore the case of the `Content-Encoding` header in the response. This was preventing responses that are gzipped from being uncompressed. I tested this with gzipped responses through the OkHttp interceptor on Android. This could potentially address #79 as I was experiencing that issue before this change.

![sonar](https://user-images.githubusercontent.com/1328587/42414919-284b80bc-8238-11e8-8cf9-d9e9b9bea133.jpg)
Pull Request resolved: https://github.com/facebook/Sonar/pull/143

Differential Revision: D8780509

Pulled By: danielbuechele

fbshipit-source-id: 1a2d86226a8a0204ff43eb5f9394a56c04fd2d8d

src/plugins/network/index.js

@@ -81,7 +81,7 @@ const COLUMNS = {
   },
 };
 
-export function getHeaderValue(headers: Array<Header>, key: string) {
+export function getHeaderValue(headers: Array<Header>, key: string): string {
   for (const header of headers) {
     if (header.key.toLowerCase() === key.toLowerCase()) {
       return header.value;