Don't use Abstract fields (#5)

Author: anriseth

src/optim_tests/multivariate/unconstrained.jl

@@ -7,8 +7,8 @@ export OptimizationProblem, objective, gradient, objective_gradient, hessian
 #######################
 # TODO: We could write fg! calls as well here
 #######################
-immutable OptimizationProblem{P, Tfg, Tf <: Real}
-    name::AbstractString
+immutable OptimizationProblem{P, Tfg, Tf <: Real, TS <: AbstractString}
+    name::TS
     f::Function
     g!::Function
     fg!::Tfg