PQTypeMap should support `get`

[samuel-massinon]

julia> get(LibPQ._DEFAULT_TYPE_MAP.type_map, 21, "temp")
Int16

julia> get(LibPQ._DEFAULT_TYPE_MAP, 21, "temp")
ERROR: MethodError: no method matching get(::LibPQ.PQTypeMap, ::Int64, ::String)
Closest candidates are:
  get(::REPL.Terminals.TTYTerminal, ::Any, ::Any) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/REPL/src/Terminals.jl:176
  get(::XMLDict.XMLDictElement, ::Any, ::Any) at /home/sam/.julia/packages/XMLDict/wy8cO/src/XMLDict.jl:35
  get(::AbstractArray, ::Integer, ::Any) at abstractarray.jl:1221
  ...
Stacktrace:
 [1] top-level scope at REPL[99]:1