Problem with short operations + underscores

Nobody has claimed this yet.

Given:

```
parser = Trollop::Parser.new do
opt :silent, %q{Causes the script to not ask any questions, instead all questions turn into email + abort; used by cron}, :short => 's'
opt :region, %q{Causes the script only work on the listed region}, :short => 'r', :type => String
opt :noinst, %q{Causes the script to skip the checking of all instances}, :short => 'i'
end
```

Running it with -i gives us:

```
{:silent=>false,
:region=>nil,
:noinst=>true,
:help=>false,
:noinst_given=>true}
```

However, given:

```
parser = Trollop::Parser.new do
opt :silent, %q{Causes the script to not ask any questions, instead all questions turn into email + abort; used by cron}, :short => 's'
opt :region, %q{Causes the script only work on the listed region}, :short => 'r', :type => String
opt :no_instances, %q{Causes the script to skip the checking of all instances}, :short => 'i'
end
```

which is what I had originally, running it with -i gives:

```
{:silent=>false,
:region=>nil,
:no_instances=>false,
:help=>false,
:no_instances_given=>true}
```

which strikes me as wrong.

Start by reproducing the two Trollop::Parser.new examples using the opt declarations and the -i argument. Trace how short options map to underscored option names, then verify that the corrected behavior sets no_instances to true and preserves the corresponding _given value.

Written by the indexing model from the issue text.