services/base: make parameter serialization of get_params customizable
services/base: make parameter serialization of get_params customizable

Some services want boolean values to be represented as "true" and
"false" while others want "1" and "0". In order to make the get_params
function more generic, make the parameter serialization routine
pluggable.

80vs90/libsaas,ducksboard/libsaas,livingbio/libsaas,CptLemming/libsaas