Remove unnecessary as.matrix() call

The period.XXX() functions (sum, prod, min, max) only work on
univariate series, so the as.matrix() call is not necessary. It makes
the functions considerably slower, because it converts the xts index
into row names for the matrix.

See #278.

R/period.R

@@ -43,7 +43,7 @@ function(x,INDEX) {
   if(ep[1L] != 0L) ep <- c(0L,ep)
   if(ep[length(ep)] != NROW(x)) ep <- c(ep,NROW(x))
 
-  xx <- as.double(as.matrix(x))
+  xx <- as.double(x)
   xa <- .Call("xts_period_sum", xx, ep, PACKAGE = "xts")
 
   if(timeBased(index(x))) {
@@ -61,7 +61,7 @@ function(x,INDEX) {
   if(ep[1] != 0L) ep <- c(0L,ep)
   if(ep[length(ep)] != NROW(x)) ep <- c(ep,NROW(x))
 
-  xx <- as.double(as.matrix(x))
+  xx <- as.double(x)
   xa <- .Call("xts_period_prod", xx, ep, PACKAGE = "xts")
 
   if(timeBased(index(x))) {
@@ -79,7 +79,7 @@ function(x,INDEX) {
   if(ep[1] != 0L) ep <- c(0L,ep)
   if(ep[length(ep)] != NROW(x)) ep <- c(ep,NROW(x))
 
-  xx <- as.double(as.matrix(x))
+  xx <- as.double(x)
   xa <- .Call("xts_period_max", xx, ep, PACKAGE = "xts")
 
   if(timeBased(index(x))) {
@@ -97,7 +97,7 @@ function(x,INDEX) {
   if(ep[1] != 0L) ep <- c(0L,ep)
   if(ep[length(ep)] != NROW(x)) ep <- c(ep,NROW(x))
 
-  xx <- as.double(as.matrix(x))
+  xx <- as.double(x)
   xa <- .Call("xts_period_min", xx, ep, PACKAGE = "xts")
 
   if(timeBased(index(x))) {