http://www.rsa.com/rsalabs/node.asp?id=2125:

NOTE: A new OID has been defined for the combination
of the v1.5 signature scheme and the SHA-224 hash function:
        sha224WithRSAEncryption OBJECT IDENTIFIER ::=
Like the other sha*WithRSAEncryption OIDs in PKCS #1 v2.1,
this OID has NULL parameters.
The DigestInfo encoding for SHA-224 (see Section 9.2, Note 1) is:
        (0x)30 2d 30 0d 06 09 60 86 48 01 65 03 04 02 04 05 00 04 1c || H

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/6208076