Fix type comparisons to library names. Closes #1158

scripts/http-grep.nse

@@ -258,7 +258,7 @@ action = function(host, port)
   end
 
   -- check if match arugment is a table or a single value
-  if match and type(match) ~= table then
+  if match and type(match) ~= 'table' then
     to_be_searched['User Pattern 1'] = {}
     table.insert(to_be_searched['User Pattern 1'], match)
   elseif type(match) == 'table' then

scripts/nbd-info.nse

@@ -100,7 +100,7 @@ local function parse_args()
     -- An empty string for an export name indicates to the server that
     -- we wish to attach to the default export.
     arg = {}
-  elseif type(arg) ~= table then
+  elseif type(arg) ~= 'table' then
     arg = {arg}
   end
   args.export_name = arg

scripts/whois-ip.nse

@@ -2223,7 +2223,7 @@ end
 
 function get_period( period )
 
-  if type( period ) ~= string or ( period == "" ) then return nil end
+  if type( period ) ~= 'string' or ( period == "" ) then return nil end
   local quant, unit = period:match( "(-?+?%d*%.?%d*)([SsMmHhDd]?)" )
   if not ( tonumber( quant ) ) then return nil end