don't add a second separator if not needed

Using let g:airline_section_y = airline#section#create_right(['ffenc','%{strftime("%H:%M")}'])
will result in an output string of `utf-8[unix] < < 00:00`

This happens, because the function util#prepend() will eventually add an
extra separator, if the width is zero. Therefore, when building the
string, remember, if the last section added an extra separator and only
add one, if there hasn't been added one before.

fixes #1220

autoload/airline/section.vim

@@ -17,6 +17,7 @@ function! s:create(parts, append)
   for idx in range(len(a:parts))
     let part = airline#parts#get(a:parts[idx])
     let val = ''
+    let add_sep = get(l:, 'add_sep', 0)
 
     if exists('part.function')
       let func = (part.function).'()'
@@ -27,7 +28,11 @@ function! s:create(parts, append)
         let val .= s:spc.g:airline_left_alt_sep.s:spc
       endif
       if a:append < 0 && idx != 0
-        let val = s:spc.g:airline_right_alt_sep.s:spc.val
+        let t = ''
+        if !add_sep
+          let t = s:spc.g:airline_right_alt_sep.s:spc
+        endif
+        let val = t.val
       endif
       if exists('part.raw')
         let _ .= s:wrap_accent(part, val.(part.raw))
@@ -42,10 +47,15 @@ function! s:create(parts, append)
 
     if a:append > 0 && idx != 0
       let partval = printf('%%{airline#util#append(%s,%s)}', func, minwidth)
+      " will add an extra separator, if minwidth is zero
+      let add_sep = (minwidth == 0)
     elseif a:append < 0 && idx != len(a:parts) - 1
       let partval = printf('%%{airline#util#prepend(%s,%s)}', func, minwidth)
+      " will add an extra separator, if minwidth is zero
+      let add_sep = (minwidth == 0)
     else
       let partval = printf('%%{airline#util#wrap(%s,%s)}', func, minwidth)
+      let add_sep = 0
     endif
 
     if exists('part.condition')

t/section.vim

@@ -72,5 +72,10 @@ describe 'section'
     let s = airline#section#create(['conditional'])
     Expect s == '%{0 ? airline#util#wrap("conditional",0) : ""}'
   end
+
+  it 'should not draw two separators after another'
+    let s = airline#section#create_right(['ffenc','%{strftime("%H:%M")}'])
+    Expect s == '%{airline#util#prepend(airline#parts#ffenc(),0)}%{strftime("%H:%M")}'
+  end
 end