Fix a couple of db.runCommand() issues

[sorribas]

This fixes 2 use cases of the db.runCommand() that didn't work on mongojs. The first was to run a database command that doesn't receive a parameter such as db.ping(). For example the following statements

db.runCommand({ping:1});
db.runCommand('ping');

Are equivalent in the mongo shell. The first worked in mongojs but the second didn't.

The other case was using a command that gets applied to a collection without parameters such as

db.runCommand('count');
db.runCommand('count', {});

These 2 lines are equivalent in the mongo shell but the first one didn't work on mongojs. This pull request fixes both this issues, adds a test case for each one and updates the docs accordingly.

[sorribas]

There you go. I think that's better.

[mafintosh]

Great stuff (as usual)!

[sorribas]

Thanks.

[mafintosh]

Released as 0.8.0