Add parameterless overloads of TrimStart/TrimEnd

[thiagocamargos]

Add parameterless overloads of TrimStart/TrimEnd on String.Manipulation.cs, model.xml and ref/mscorlib.cs

Fix https://github.com/dotnet/corefx/issues/6485

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[joperezr]

Hello @thiagocamargos thanks for your contribution!

LGTM apart from that small comment I gave. We will need to add the appropiate tests into corefx once we expose this.

[joperezr]

Nit: You could simplify this by simply doing:

public string TrimStart() => TrimHelper(TrimHead);
public string TrimEnd() => TrimHelper(TrimTail);

[thiagocamargos]

Hi @joperezr
I see there are already some tests on https://github.com/dotnet/corefx/blob/master/src/System.Runtime/tests/System/StringTests.cs#L2346 and https://github.com/dotnet/corefx/blob/master/src/System.Runtime/tests/System/StringTests.cs#L2328
that used the TrimStart/End(params char []) overload and will use now the parameterless overload.

Should I adapt the test to ensure it will also keep calling the params char [] overload when trimChars is null?

[joperezr]

Should I adapt the test to ensure it will also keep calling the params char [] overload when trimChars is null

Yes, I believe you should ensure it keeps calling the other params one so that we don't loose coverage there and perhaps add some basic ones for this two methods as well.

[joperezr]

@thiagocamargos thanks for your contribution!

[jkotas]

@thiagocamargos Would you mind porting this change to CoreRT (https://github.com/dotnet/corert/tree/master/src/System.Private.CoreLib/src) as well?

[thiagocamargos]

Hi @jkotas
Sure, I'll do that.

[justinvp]

@thiagocamargos, I included these in dotnet/corert#2587 while I was porting my changes from #9009. Apologies if you were already working on it.

[thiagocamargos]

@justinvp no problem, I could not work on last couple weeks