ReadCsv cannot set empty string as missingValues

[zyzhu]

Repro steps sample.csv, c3 column is all empty

row,c1,c2,c3
1,,5,
2,4,6,

The following lines treat `` as missing value. It then access the row by key.

[<Literal>]
let sample = "C:/FSharp/sample.csv"
let r = Frame.ReadCsv(sample,missingValues=[|""|]).IndexRows<int>("row")
r.Rows.[2].As<float>()

Expected outcome

val r : Frame<int,string> =

     c1        c2 c3        
1 -> <missing> 5  <missing> 
2 -> 4         6  <missing> 

val it : Series<string,float> =

c1 -> 4         
c2 -> 6         
c3 -> <missing> 

Actual outcome

val r : Frame<int,string> =
     c1        c2 c3 
1 -> <missing> 5      
2 -> 4         6     

System.FormatException: Input string was not in a correct format.

Suggestion Empty string cannot be set as missing values because of the following line. c3 column is inferred to be string even though it's set as one of the missingValues. https://github.com/fsharp/FSharp.Data/blob/master/src/Csv/CsvInference.fs#L130

Wait till FSharp.Data address the following issue https://github.com/fsharp/FSharp.Data/issues/1192