Allow to parse double-float in JSON.

[fukamachi]

float in Common Lisp usually means single-float and it can represent a number which has 8 digits at most.
However, JSON's float can be a double-float:

(jojo:parse "35.65910807942215")
;=> 35.659107

This patch allows jojo:parse to parse double-floats too:

(jojo:parse "35.65910807942215")
;=> 35.65910807942215d0

(jojo:parse "35.659108")
;=> 35.659108

[coveralls]

Coverage remained the same at 71.667% when pulling fcfceee on fukamachi:parse-double-float into fba9d5e on Rudolph-Miller:master.

[coveralls]

Coverage remained the same at 71.667% when pulling fcfceee on fukamachi:parse-double-float into fba9d5e on Rudolph-Miller:master.

[rudolph-miller]

Thanks!

[fukamachi]

The test failed with ABCL.
https://travis-ci.org/Rudolph-Miller/jonathan/jobs/169708369#L947-L948

It seems the numbers after the decimal point is bigger than the range of ABCL's fixnum.

(jojo:parse "35.65910807942215")
;=> 35.00000239817799d0

most-positive-fixnum
;=> 2147483647

(typep 65910807942215 'fixnum)
;=> NIL

Perhaps the type specification at (the fixnum (parse-integer rest-num-str)) should be omitted.
https://github.com/Rudolph-Miller/jonathan/blob/master/src/decode.lisp#L272

[rudolph-miller]

Fixed. a163a0f