The Content-Type for TextMessageEncoder doesn't support a whitespace character between content-type and charset, moreover the IsContentTypeSupported overload doesn't work

**Describe the bug**
Trying to connect a WCF client to a Java SOAP receiver, however we are having an issue with the content type.

By design or not, the Content-Type for TextMessageEncoder doesn't support a whitespace character between content-type and charset as is easily identifiable in the relevant code:
[https://github.com/dotnet/wcf/blob/main/src/System.Private.ServiceModel/src/System/ServiceModel/Channels/TextMessageEncoder.cs#L359](https://github.com/dotnet/wcf/blob/main/src/System.Private.ServiceModel/src/System/ServiceModel/Channels/TextMessageEncoder.cs#L359)

So instead we tried to use a CustomTextMessageEncoder and override IsContentTypeSupported which was mentioned as a fix by many, but the overridden method is never hit and the connection still fails on the following error:

**Error message:**
```
The content type application/soap+xml;charset=utf-8 of the response message does not match the content type of the binding (application/soap+xml; charset=utf-8). If using a custom encoder, be sure that the IsContentTypeSupported method is implemented properly.
```

**To Reproduce**
Steps to reproduce the behavior:
1. Setup a client binding expecting content type "application/soap+xml; charset=utf-8"
2. Setup a server binding expecting content type "application/soap+xml;charset=utf-8"
3. Have them communicate with eachother
(In our case it was the response message of the server that resulted in a content type mismatch probably because their soap implementation doesn't have this limitation, in case of a WCF to WCF implementation the error might already pop up earlier)

*optional steps for IsContentTypeSupported override issue*
4. Create a CustomTextMessageEncoder as per the microsoft example:
[https://docs.microsoft.com/en-us/dotnet/framework/wcf/samples/custom-message-encoder-custom-text-encoder](https://docs.microsoft.com/en-us/dotnet/framework/wcf/samples/custom-message-encoder-custom-text-encoder)
5. Override _public virtual bool IsContentTypeSupported(string contentType)_, set a breakpoint in the new version and watch the breakpoint never be hit, the method is never used.

**Callstack of the error Message:**
`System.ServiceModel.ProtocolException:
at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage (mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089)
at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke (mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089)
at IrrelevantInternalCode`

**Expected behavior**
The space should be supported or it should be possible to override the default behavior using the IsContentTypeSupported overload