Fix #382. Remove spurious method call error in function definition.

[zengmao]

Fixes #382 with a single small change in src/linting/checks.jl. In test/runtest.jl, I've added the following new test which only passes with the new code:

cst = parse_and_pass("""
function f(a::T, invert=false)::T where {T <: Integer}
    invert ? -a : a
end""")
@test !StaticLint.haserror(cst.args[1].args[1].args[1].args[1])