fread should un-escape escaped quotes in fields

Start by reproducing the issue with fread using the tbl1.csv and tbl2.csv examples in the report, then trace the fread parsing path for quoted fields. Compare the escaped-quote and doubled-quote cases. Done means fread reads tbl1.csv successfully and returns the same unescaped field values as read.table.

According to the docs:

A quoted field must start with quote and end with a quote that is also immediately followed by sep or \n. Thus, unescaped quotes may be present in a quoted field (...,2,"Joe, "Bloggs"",3.14,...) as well as escaped quotes (...,2,"Joe \",Bloggs\"",3.14,...).

The following csv should be supported by fread.

library(data.table)
dt <- data.table(a = 1:2, b = c('f(c("a","b"))','sum(1,2)'))

dt
#    a             b
#1: 1 f(c("a","b"))
#2: 2      sum(1,2)

write.table(dt,"tbl1.csv",sep=",",na="",col.names=TRUE,row.names=FALSE,qmethod="escape")
write.table(dt,"tbl2.csv",sep=",",na="",col.names=TRUE,row.names=FALSE,qmethod="double")

system("cat tbl1.csv")
# "a","b"
#1,"f(c(\"a\",\"b\"))"
#2,"sum(1,2)"
system("cat tbl2.csv")
# "a","b"
#1,"f(c(""a"",""b""))"
#2,"sum(1,2)"

# output NA
fread("tbl1.csv",sep=",")
# Error in fread("tbl1.csv", sep = ",") : 
#   Expected sep (',') but new line, EOF (or other non printing character) ends field 1 when detecting types (   first): 2,"sum(1,2)"
# In addition: Warning message:
#   In fread("tbl1.csv", sep = ",") :
#   Starting data input on line 2 and discarded previous non-empty line: "a","b"

# incorrect output
fread("tbl2.csv",sep=",")
# a                 b
#1: 1 f(c(""a"",""b""))
#2: 2          sum(1,2)

# incorrect output
read.table("tbl1.csv",sep=",",header=TRUE)
#   a                 b
#1 1 f(c(\\a\\,\\b\\))
#2 2          sum(1,2)

# correct output
read.table("tbl2.csv",sep=",",header=TRUE)
# a               b
#1 1 f(c("a","b"))
#2 2      sum(1,2)

Findings:
as of now, only writing using qmethod="double" and read.table correctly supports write-read such kind of data.

Latest dev data.table, my locale, etc:

> sessionInfo()
# R version 3.1.3 (2015-03-09)
# Platform: x86_64-pc-linux-gnu (64-bit)
# Running under: Ubuntu 14.04.2 LTS
# 
# locale:
#  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_DK.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=C             
#  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
# 
# attached base packages:
# [1] stats     graphics  grDevices utils     datasets  methods   base     
# 
# other attached packages:
# [1] data.table_1.9.5
# 
# loaded via a namespace (and not attached):
#  [1] bitops_1.0-6   chron_2.3-45   devtools_1.7.0 evaluate_0.5.5 formatR_1.0    httr_0.6.1     knitr_1.8      RCurl_1.95-4.5 stringr_0.6.2  tools_3.1.3