Attempt to fix JSON parsing of text/plain content type (#1172)

* UseContentType

* Fix unit tests

* Add a unit test and an integration test for the fix.

* Simplify body type checking with GetBodyType extension.

* Split IBodyDataExtension, and use imperative style instead of functional style

* Remove excessive null forgiving operators

* Adjust braces

---------

Co-authored-by: Ruxo Zheng <rz@just.sent.as>

src/WireMock.Net.Abstractions/Models/IBodyDataExtension.cs

@@ -0,0 +1,20 @@
+using WireMock.Types;
+
+// ReSharper disable once CheckNamespace
+namespace WireMock.Util;
+
+public static class IBodyDataExtension
+{
+    public static BodyType GetBodyType(this IBodyData bodyData)
+    {
+        if (bodyData.DetectedBodyTypeFromContentType is not null and not BodyType.None)
+        {
+            return bodyData.DetectedBodyTypeFromContentType.Value;
+        }
+        if (bodyData.DetectedBodyType is not null and not BodyType.None)
+        {
+            return bodyData.DetectedBodyType.Value;
+        }
+        return BodyType.None;
+    }
+}