Fix content-length-header (#110)

Author: donny-dont

lib/src/io_client.dart

@@ -31,9 +31,6 @@ class IOClient extends BaseClient {
       ioRequest
           ..followRedirects = context['io.followRedirects'] ?? true
           ..maxRedirects = context['io.maxRedirects'] ?? 5
-          ..contentLength = request.contentLength == null
-              ? -1
-              : request.contentLength
           ..persistentConnection = context['io.persistentConnection'] ?? true;
       request.headers.forEach((name, value) {
         ioRequest.headers.set(name, value);

test/client_test.dart

@@ -330,6 +330,7 @@ void main() {
             'method': 'DELETE',
             'path': '/',
             'headers': {
+              'content-length': ['0'],
               'user-agent': [userAgent()],
               'x-random-header': ['Value'],
               'x-other-header': ['Other Value']