SPRINGFIELD — Gov. Bruce Rauner will sign a law to make it more difficult to round up undocumented immigrants, according to a coalition of supporters.Pro-immigration groups who back the idea said this week that Illinois’ governor “has committed” to sign Senate Bill 31, a Democrat-sponsored measure that says local law enforcement agencies can’t detain people on behalf of federal officials unless they have a warrant signed by a judge.A spokeswoman in Rauner’s office did not confirm or deny …