Add additional methods to rescale and rescale_mid

[karawoo]

#75 didn't include a rescale.NULL method, which is sometimes needed. For example this:

library("ggplot2")
ggplot(mtcars, aes(cyl, mpg)) +
  geom_point() +
  scale_x_continuous(breaks = NULL)

should create a plot like:

But currently it throws Error in UseMethod("rescale"): no applicable method for 'rescale' applied to an object of class "NULL" instead. The lack of rescale.NULL also broke some code in the ggplot2-specs vignette.

[zeehio]

Thanks @karawoo. Do you think scales needs to provide a rescale_mid.NULL method as well?

[hadley]

Looks good. As @zeehio suggests, can you please also add rescale_mid.NULL?

[karawoo]

Will do. We also need logical methods for this test to pass.