Expose EnableUnsecuredResponse in SecurityBindingElement and add unit test

[imcarolwang]

Relates to #4817, #3705.

Note:
#4817 mentioned to expose both AllowInsecureTransport & EnableUnsecuredResponse, this PR only exposes EnableUnsecuredResponse since the impementation of property AllowInsecureTransport doesn't exist in the class on WCF core, it needs to be discussed whether to add it.

[HongGit]

@imcarolwang can you please create a TWMC and send a invalid credentials? Service will reply a fault, toggle the value of the property and you should get two different exceptions.

[imcarolwang]

@imcarolwang can you please create a TWMC and send a invalid credentials? Service will reply a fault, toggle the value of the property and you should get two different exceptions.

I don't quite understand what TWMC stands for, I added a scenario test base on my understanding of the comments, and for different settings of EnableUnsecuredResponse the test does get different exception when sending invalid client credential.

[mconnew]

TWMC means SecurityMode.TransportWithMessageCredentials. The custom binding you used is equivalent to what you can get from using TWMC so your test matches what was asked.