Keys with colons causes crash

[bkeroackdsc]

Version 0.5

Example:

gof3r put -b "mybucket" -k "ThuDec1519:13:41UTC2016.tar" < "/tmp/file.tar"
panic: runtime error: slice bounds out of range

goroutine 1 [running]:
github.com/rlmcpherson/s3gof3r.(*signer).buildCanonicalString(0xc8200452d8)
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/sign.go:120 +0x4fb
github.com/rlmcpherson/s3gof3r.(*signer).sign(0xc8200452d8)
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/sign.go:66 +0x63
github.com/rlmcpherson/s3gof3r.(*Bucket).Sign(0xc82000e980, 0xc8200d6000)
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/sign.go:59 +0x1e5
github.com/rlmcpherson/s3gof3r.(*putter).retryRequest(0xc820001080, 0x7fca00, 0x4, 0xc82000e9c0, 0x1e, 0x0, 0x0, 0xc820011d40, 0x0, 0x0, ...)
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/putter.go:382 +0x37e
github.com/rlmcpherson/s3gof3r.newPutter(0x7fe8d8, 0x5, 0x7fffc140f6f0, 0x10, 0x0, 0xc820011dd0, 0x30, 0xc820013f70, 0x1, 0x0, ...)
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/putter.go:88 +0x349
github.com/rlmcpherson/s3gof3r.(*Bucket).PutWriter(0xc8200459e0, 0x7fffc140f6e5, 0x1b, 0xc820011d40, 0xc820045a90, 0x0, 0x0, 0x0, 0x0)
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/s3gof3r.go:135 +0x126
main.(*putOpts).Execute(0x9d5a00, 0xc82008ea20, 0x0, 0x9, 0x7f81b8ff8590, 0xc820011da0)
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/gof3r/put.go:53 +0x65f
github.com/jessevdk/go-flags.(*Parser).ParseArgs(0xc8200784b0, 0xc82000a0b0, 0x9, 0x9, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/rm/src/github.com/jessevdk/go-flags/parser.go:301 +0xa16
github.com/jessevdk/go-flags.(*Parser).Parse(0xc8200784b0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/rm/src/github.com/jessevdk/go-flags/parser.go:175 +0x9b
main.main()
	/Users/rm/src/github.com/rlmcpherson/s3gof3r/gof3r/main.go:61 +0x191