deps: Update http to 1.6.0, from 1.5.0

Changelog:
  https://pub.dev/packages/http/changelog#160

One breaking change which now omits `charset` parameter for all
media types except for `text` and some `application/xml` types:
  https://github.com/dart-lang/http/blob/a22386e9c/pkgs/http/lib/src/request.dart#L18-L31
  dart-lang/http#1798

That breaks some of our tests which expect `charset=utf-8` being
present in the `Content-Type` header, so this commit includes a fix
for those tests, simply removing that expectation.

The default charset still remains `utf-8`:
  https://github.com/dart-lang/http/blob/a22386e9c/pkgs/http/lib/src/request.dart#L179

Author: rajveermalviya

pubspec.lock

@@ -509,10 +509,10 @@ packages:
     dependency: "direct main"
     description:
       name: http
-      sha256: bb2ce4590bc2667c96f318d68cac1b5a7987ec819351d32b1c987239a815e007
+      sha256: "87721a4a50b19c7f1d49001e51409bddc46303966ce89a65af4f4e6004896412"
       url: "https://pub.dev"
     source: hosted
-    version: "1.5.0"
+    version: "1.6.0"
   http_multi_server:
     dependency: transitive
     description:

test/api/core_test.dart

@@ -137,7 +137,7 @@ void main() {
             ...authHeader(email: eg.selfAccount.email, apiKey: eg.selfAccount.apiKey),
             ...kFallbackUserAgentHeader,
             if (expectContentType)
-              'content-type': 'application/x-www-form-urlencoded; charset=utf-8',
+              'content-type': 'application/x-www-form-urlencoded',
           })
           ..body.equals(expectedBody);
       }));
@@ -213,7 +213,7 @@ void main() {
             ...authHeader(email: eg.selfAccount.email, apiKey: eg.selfAccount.apiKey),
             ...kFallbackUserAgentHeader,
             if (expectContentType)
-              'content-type': 'application/x-www-form-urlencoded; charset=utf-8',
+              'content-type': 'application/x-www-form-urlencoded',
           })
           ..body.equals(expectedBody);
       }));
@@ -244,7 +244,7 @@ void main() {
             ...authHeader(email: eg.selfAccount.email, apiKey: eg.selfAccount.apiKey),
             ...kFallbackUserAgentHeader,
             if (expectContentType)
-              'content-type': 'application/x-www-form-urlencoded; charset=utf-8',
+              'content-type': 'application/x-www-form-urlencoded',
           })
           ..body.equals(expectedBody);
       }));