_₂F₁ errors when a == c and z > 1

[simeonschaub]

The following throws a DomainError, it's in a different branch than #75 though:

julia> _₂F₁(-1, 0, -1, 2.0)
ERROR: DomainError with -2.0:
log1p was called with a real argument < -1 but will only return a complex result if called with a complex argument. Try log1p(Complex(x)).
Stacktrace:
 [1] throw_complex_domainerror_neg1(f::Symbol, x::Float64)
   @ Base.Math ./math.jl:37
 [2] log1p(x::Float64)
   @ Base.Math ./special/log.jl:364
 [3] _₂F₁(a::Int64, b::Int64, c::Int64, z::Float64; method::Symbol, kwds::@Kwargs{})
   @ HypergeometricFunctions ~/.julia/packages/HypergeometricFunctions/QX81g/src/gauss.jl:11
 [4] _₂F₁(a::Int64, b::Int64, c::Int64, z::Float64)
   @ HypergeometricFunctions ~/.julia/packages/HypergeometricFunctions/QX81g/src/gauss.jl:6
 [5] top-level scope
   @ REPL[29]:1

I believe this is still well-defined, Mathematica just evaluates it to 1