Added a test for the behaviour for an update without mirror

https://code.google.com/p/cyg-apt/issues/detail?id=18

Reported by rustysco...@gmail.com, Apr 11, 2014

What steps will reproduce the problem?
1. downloag cyg-apt
2. attempt to install cygapt or run cygapt

What is the expected output? What do you see instead?
cygapt should return after setup without a problem but with this current bug, cyg-apt is entirely dysfunctional.
cyg-apt returns the following

python2.7 ./cyg-apt search rxtv
Traceback (most recent call last):
  File "./cyg-apt", line 1581, in <module>
    main()
  File "./cyg-apt", line 1546, in main
    cas.update(main_cyg_apt_rc, main_verify, main_mirror = main_mirror)
  File "./cyg-apt", line 205, in update
    if not mirror[-1] == "/":
IndexError: string index out of range

What version of the product are you using? On what operating system?
1.0.7-1
Fixed the behaviour for an update without mirror
Merge pull request #6 from alquerci/fix/update/without_mirror

Fixed the behaviour for an update without mirror