Update Bid & Ask fields to handle missing entries

Similar to Vol & OI changes implemented previously. There are requests that are failing due to the bid & ask fields not having any info. This change solves.

Author: jrburl

R/getOptionChain.R

@@ -24,8 +24,8 @@ getOptionChain.yahoo <- function(Symbols, Exp, ...)
     d <- with(x, data.frame(Strike=strike, 
                             Last=lastprice, 
                             Chg=change,
-                            Bid=bid, 
-                            Ask=ask, 
+                            Bid= if("bid" %in% names(x)) {bid} else {NA}, 
+                            Ask= if("ask" %in% names(x)) {ask} else {NA},    
                             Vol= if("volume" %in% names(x)) {volume} else {NA}, 
                             OI= if("openinterest" %in% names(x)) {openinterest} else {NA},
                             LastTradeTime= if("lasttradedate" %in% names(x)) {lasttradedate} else {NA},