PPDValue::m_aValue is simply an optional parameter description, whereas
m_aOption is the actual option being passed to cups.

For example, for the key "PageSize" (which was previously passed
without problem), a typical PPDValue could be:
m_aOption = "A4"
m_aValue = "<</PageSize[595.3 841.9]/ImagingBBox null>>setpagedevice"

However for the key "InputSlot" (which was previously not passed
to the printer), a typical PPDValue could be:
m_aOption = "RearPaperFeedSlot"
m_aValue = ""

Change-Id: I7959317c9f9d67bfafd911e710927a70edfa8792