Although former JDL leader Irv Rubin was believed yesterday to have been brain-dead, an improvement in his condition has been noted, and family members request that prayers be recited on his behalf. His Jewish name is Yisrael ben Devorah. Rubin was imprisoned almost a year ago on charges of conspiracy to attack a mosque and a Congressman's office. American law enforcement officials reported yesterday that Rubin had tried to take his own life by cutting his throat, but close friends, colleagues and relatives reject such claims as baseless. They are demanding an investigation into the circumstances of Rubin's wounds.