Cache default_ua on package load

The default_ua function is called on every request and involves
calls to the slow base function packageDescription.  Depending on the
use, this can account for up to 40% of the client-side CPU time which
is a lot when using over a loopback connection (e.g. etcd, RNeo4j).

Because this information is unlikely to change during a session (and is
infomative for the server and likely ignored anyway) computing it once
and reusing it seems safe).

By @richfitz, closes #322.

NEWS.md

@@ -1,5 +1,9 @@
 # httr 1.1.0.9000
 
+* The default user agent string is now computed once and cached. This 
+  is a small performance improvement, but important for local connections
+  (#322, @richfitz).
+
 * New `encode = "raw"` allows you to do your own encoding for requests with
   bodies.
 

R/config.r

@@ -130,13 +130,18 @@ curl_docs <- function(x) {
   BROWSE(url)
 }
 
+cache <- new.env(parent = emptyenv())
+cache$default_ua <- NULL
+
 default_ua <- function() {
-  versions <- c(
-    libcurl = curl::curl_version()$version,
-    `r-curl` = as.character(utils::packageVersion("curl")),
-    httr = as.character(utils::packageVersion("httr"))
-  )
-  paste0(names(versions), "/", versions, collapse = " ")
+  if (is.null(cache$default_ua)) {
+    versions <- c(
+      libcurl = curl::curl_version()$version,
+      `r-curl` = as.character(utils::packageVersion("curl")),
+      httr = as.character(utils::packageVersion("httr")))
+    cache$default_ua <- paste0(names(versions), "/", versions, collapse = " ")
+  }
+  cache$default_ua
 }
 
 #' Set (and reset) global httr configuration.