In Format::forge()->to_csv(), escape enclosure by a double enclosure

... not a backslash

First, in the [CSV RFC, section 2.7](http://tools.ietf.org/html/rfc4180#section-2) (it's not a specification of an Internet standard but it documents the format used for Comma-Separated Values)

_If double-quotes are used to enclose fields, then a double-quote appearing inside a field must be escaped by preceding it with another double quote.  For example:_

     "aaa","b""bb","ccc"

I've tested opening a file generating by Format->to_csv() in Excel 2000 on Windows and Open-office on Ubuntu, in the two cases, columns containing double-quote escaped by a backslash are not well parsed.

Sample:

    "aaa","b""bb","ccc"
    "aaa","b\"bb","ccc"
    "ddd","eee","fff"
    "ggg","h\"hh","iii"

Result in Open-office:

    aaa    b"bb    ccc             
    aaa    b\bb"   ccc"ddd"    eee fff"ggg"    h\hh"   iii

First line is OK. Lines 2, 3, 4 are merged.

Result in Excel 2000:

    aaa    b"bb    ccc
    aaa    b\bb"   ccc
    ddd    eee     fff
    ggg    h\hh"   iii

First line is OK. The result is better for lines 2 and 4, but their second column are not well parsed.