PERCENTILE panics database when run on integer values

[beckettsean]

Series exists and has data:

> select * from foobar
name: foobar
tags: key=val1
time                value
----                -----
2015-06-18T19:09:52.947299286Z  1
2015-06-18T19:10:01.619062526Z  2
2015-06-18T19:10:03.171388731Z  3


name: foobar
tags: key=val2
time                value
----                -----
2015-06-18T19:10:06.339544273Z  3
2015-06-18T19:10:07.611584869Z  4
2015-06-18T19:10:08.763866774Z  5

PERCENTILE query panics the database

> select percentile(value, 30) from foobar
ERR: Get http://localhost:8086/query?db=mydb&q=select+percentile%28value%2C+30%29+from+foobar: EOF

log trace

panic: interface conversion: interface is int64, not float64

goroutine 90 [running]:
github.com/influxdb/influxdb/influxql.func·021(0xc2080e8540, 0x1, 0x1, 0x0, 0x0)
        /private/tmp/influxdb20150611-31144-myuhfv/influxdb-0.9.0/src/github.com/influxdb/influxdb/influxql/functions.go:1061 +0x2ea
github.com/influxdb/influxdb/influxql.(*MapReduceJob).processAggregate(0xc2081bb0a0, 0xc208136750, 0xc2080e8530, 0xc20815cba0, 0x1, 0x1, 0x0, 0x0)
        /private/tmp/influxdb20150611-31144-myuhfv/influxdb-0.9.0/src/github.com/influxdb/influxdb/influxql/engine.go:784 +0x33d
github.com/influxdb/influxdb/influxql.(*MapReduceJob).Execute(0xc2081bb0a0, 0xc208148cc0, 0x0)
        /private/tmp/influxdb20150611-31144-myuhfv/influxdb-0.9.0/src/github.com/influxdb/influxdb/influxql/engine.go:170 +0x8ca
github.com/influxdb/influxdb/influxql.(*Executor).execute(0xc20803fc00, 0xc208148cc0)
        /private/tmp/influxdb20150611-31144-myuhfv/influxdb-0.9.0/src/github.com/influxdb/influxdb/influxql/engine.go:926 +0xba
created by github.com/influxdb/influxdb/influxql.(*Executor).Execute
        /private/tmp/influxdb20150611-31144-myuhfv/influxdb-0.9.0/src/github.com/influxdb/influxdb/influxql/engine.go:905 +0x5a

[beckettsean]

Verified it only panics if the field values are int64. If they are float64 it works fine:

> insert bat fkey=3.2
> insert bat fkey=3.2
> insert bat fkey=3.2

> insert baz fkey=3
> insert baz fkey=3
> insert baz fkey=3

> select percentile(fkey, 50) from bat
name: bat
---------
time            percentile
1970-01-01T00:00:00Z    3.2

> select percentile(fkey, 50) from baz
ERR: Get http://localhost:8086/query?db=mydb&q=select+percentile%28fkey%2C+50%29+from+baz: EOF

[jwilder]

Verified fixed by #2917