Disallow Vector{Union{String, Symbol}} to have both types in a single vector

[siddjain444]

This signature is used in many getindex() functions. There are two options to solve this:

• Create separate functions to handle Symbol and String types respectively.
• Throw an error if both types are present in the input argument. This is what DataFrames.jl does.

[chiraganand]

* Throw an error if both types are present in the input argument. This is what DataFrames.jl does.

My preference is with this because it will be easier to maintain by having less number of functions which behave exactly same except for the argument type. Plus, DataFrames.jl also follows the same approach so it is better to be consistent.