Fix double expansion of AggregateException.Flatten message

[pentp]

A by-product of fixing https://github.com/dotnet/corefx/issues/5364 was that AggregateException.Flatten() will construct a new exception using the expanded message from the existing exception which will be expanded again when used.

The test diff for this change would have been the following:

-Assert.Equal("message (A) (B) (C) (A) (B) (C)", flattened1.Message);
+Assert.Equal("message (A) (B) (C)", flattened1.Message);
-Assert.Equal("message (message (A) (B) (C)) (message (A) (B) (C)) (A) (B) (C) (A) (B) (C)", flattened2.Message);
+Assert.Equal("message (A) (B) (C) (A) (B) (C)", flattened2.Message);

@stephentoub Is the check for exact type match OK or too much?

[stephentoub]

Seems reasonable. Thanks.

However, the dotnet/runtime repo is still being enabled and we don't yet have the corefx tests running against a live built corelib; at this point, although they're in the same repo, we're still running the tests against a nuget package containing corelib. So the tests will need to be submitted in a separate PR.

[pentp]

Maybe it's better to wait for #71 to complete first. This PR could then be completed in one step?

[stephentoub]

Whichever you prefer.

[pentp]

Created a new PR for this as the runtime repo was made public.