Conditioning fails with tuple of pair

[penelopeysm]

julia> @model function g()
           x ~ Normal()
           y ~ Normal()
       end
g (generic function with 2 methods)

julia> g() | (@varname(x) => 1.0, @varname(y) => 2.0)
ERROR: MethodError: no method matching _make_conditioning_values(::Tuple{Pair{VarName{…}, Float64}, Pair{VarName{…}, Float64}})
The function `_make_conditioning_values` exists, but no method is defined for this combination of argument types.

I probably introduced this in #776