Return of income, etc., not to be invalid on certain grounds

No return of income, assessment, notice, summons or other proceeding, furnished or made or issued or taken, or purported to have been furnished or made or issued or taken, in pursuance of any of the provisions of this Act, shall be invalid or shall be considered to be invalid merely by reason of any mistake, defect or omission in such return of income, assessment, notice, summons or other proceeding, if such return of income, assessment, notice, summons or other proceeding is in substance and effect in conformity with or according to the intent and purposes of this Act.