fix(http): correctly set content type for POST requests

lib/modules/dashboard/core/api/api.dart

@@ -48,6 +48,7 @@ class API {
           'start': _startDate(today),
           'end': _endDate(today),
         },
+        contentType: Headers.jsonContentType,
         responseType: ResponseType.json,
         headers: profile.lidarrHeaders,
         followRedirects: true,
@@ -92,6 +93,7 @@ class API {
           'start': _startDate(today),
           'end': _endDate(today),
         },
+        contentType: Headers.jsonContentType,
         responseType: ResponseType.json,
         headers: profile.radarrHeaders,
         followRedirects: true,
@@ -147,6 +149,7 @@ class API {
           'start': _startDate(today),
           'end': _endDate(today),
         },
+        contentType: Headers.jsonContentType,
         responseType: ResponseType.json,
         headers: profile.sonarrHeaders,
         followRedirects: true,

lib/modules/lidarr/core/api/api.dart

@@ -14,6 +14,7 @@ class LidarrAPI {
         queryParameters: {
           if (profile.lidarrKey != '') 'apikey': profile.lidarrKey,
         },
+        contentType: Headers.jsonContentType,
         responseType: ResponseType.json,
         headers: LunaProfile.current.lidarrHeaders,
         followRedirects: true,

lib/modules/nzbget/core/api/api.dart

@@ -17,6 +17,7 @@ class NZBGetAPI {
         headers: profile.nzbgetHeaders,
         followRedirects: true,
         maxRedirects: 5,
+        contentType: Headers.jsonContentType,
         responseType: ResponseType.json,
       ),
     );

lib/modules/sabnzbd/core/api/api.dart

@@ -18,6 +18,7 @@ class SABnzbdAPI {
         headers: profile.sabnzbdHeaders,
         followRedirects: true,
         maxRedirects: 5,
+        contentType: Headers.jsonContentType,
         responseType: ResponseType.json,
       ),
     );