feature: added test case to acronym exercise to cover missed case

[Llcoolsouder]

The current iteration of the acronym exercise missed the case where the first letter of the phrase is lowercase.

I had a mentee start their solution with

std::string result(1, input[0]);

and all tests passed.

My new test accounts for this situation.

[siebenschlaefer]

The C++ track (usually) just translates the test cases from the language-agnostic problem specification to C++.
The best place to suggest changes like this is at github.com/exercism/problem-specifications/issues.

BTW: Two somehow related suggestions already exist: #1431 and #1432.

[kytrinyx]

@Llcoolsouder Thanks for the suggestion!

If you would like to propose these changes in problem-specifications, please open an issue there.
If they get accepted upstream, then we will update the test suite here to include them.

I'm going to go ahead and close this.